It would require voting for the other party. The big issue is the parties being willing to back aging candidates. But of course, party leaders are just as old -- so they see no problem.
There's more than two parties. Nobody is forcing you to vote for one of them. Parties are also an anti-pattern. Again we need a better system than first past the post that naturally biases towards a two party system.
Party primaries devolve into 'the devil you know vs the devil you dont' and the Devil You Know usually has a boatload of cash to spend on the campaign that the Devil You Dont doesn't have. You are right of course, the solution lies in eliminating the party system and implementing a ranked voting system of some kind.
Due to practicality and congressional gridlock, this has to happen state by state, and in many states the 'easiest' method is some sort of ballot initiative. I encourage anyone who reads this to look up the process in their state (https://ballotpedia.org/List_of_ballot_measures_by_state) and at least try to get a signature out to one so they show up on the ballot.
All other things being equal it is in their best interest to re-elect the incumbent. Seniority and connections figure into what posts you get and what influence you have. McConnell's and Feinstein's constituents have an outsized influence on the proceedings. Why would Kentuckians cede control of the Senate in favor of a politician, no matter how skilled, who had zero seniority?
I'm not especially in favor of short term limits, since there is a benefit to politicians with experience. But I'd definitely consider options that better balance their reasonable desire to elect experienced and influential candidates with the very heavy hand that experience puts on everybody else.
And voting for the party is sensible. Most important votes happen along party lines. Including the most important, for Speaker and Majority Leader, who set the agenda.
Most other votes will be largely sublimated to the party priorities. The individuals will determine the specific contents of what they're voting on, but in general the person from the other party is going to vote against the things you're most interested in. You may watch your party vote on things that aren't your favorite, but it's unlikely they'll be repugnant to you. That's how they got to be your allies.
If you want to get more involved than that, you show up for the primaries. That's a multi-way contest to find the person who best represents you and your allies. That's where the party priorities get set. People who vote only in the general election are missing out on the real work of politics.